In numerous industrial applications it is necessary to remove particulates from a hot-gas stream before the gas is discharged into the atmosphere, subjected to chemical treatment, is recycled for reuse, etc. The particles may, in turn, be subjected to a subsequent treatment, e.g., to convert them into a useful product, can be recycled to an earlier stage in chemical, metallurgical and industrial processing, can be recovered for reuse without modification, or can be discarded.
The removal of particulates from a hot-gas stream is exemplified in the treatment of the hot gases produced by plants for the production of cement clinker by the dry process. In such plants, the cement components are passed through a kiln in which the composition is brought into contact with a hot gas, generally produced by combustion of a fuel, the cement clinker being removed from the kiln. The exhaust gases from the kiln are at a high temperature and entrain particulates which cannot be released into the environment with the exhaust gas, so that processes have been developed for the removal of the particles from the gas.
The removed particles can be recycled to an earlier stage in a cement plant since they contain various useful cement components. A cement plant may also give rise to hot gases entraining particulates of this type at other stages, e.g., dryers, mills.
It is the common practice to recover the particulates from the hot gases entraining same at least in part with the aid of electrostatic precipitators. As is well known, the electrostatic precipitation process produces, by corona discharge in the gas, an electrical charge on the solid particles which are entrained therewith, the particles being attracted to oppositely charged collector electrodes which may be rapped from time to time to deposit the accumulated particles in dust bins or the like from which the particles may be withdrawn.
It has also been recognized for some time, in connection with the use of electrostatic precipitators, that it is desirable to maintain the humidity of the gas entering the electrostatic precipitator above a certain level for optimum operation of the electrostatic precipitator. In other words, with moisture contents below a predetermined level, the dust-removal efficiency of the electrostatic precipitator falls and it is possible that an electrostatic precipitator of a given capacity may not be able to process completely all of the gas which must be treated, gas which contains an excessive proportion of entrained particles, or particulate-containing gases of fluctuating throughput.
Thus it has been proposed heretofore to "condition" the gases entering a system for the electrostatic removal of particulates from the gas by increasing the moisture content thereof.
In this connection it is known to provide upstream of an electrostatic precipitator or group of electrostatic precipitators, a conditioning unit which can be used to increase the moisture content of the particulate-containing gas stream. It has been found to be advantageous to combine this moisturezation unit with a cooling tower in which the temperature of the gas is reduced. In such a tower, the gas stream may be passed along a vertical path which can be controlled by elements within the tower to minimize turbulence, while being subjected to treatment with a fine water spray. The water is thus atomized directly into the gas stream within this tower.
As a result, the gas emerging from the tower has an increased dewpoint, humidity or moisture content and a lower temperature, efforts being made within the tower to ensure homogeneous distribution of the moisture in the gas.
Invariably in such a tower, moist particulates, i.e., dusts, are settled out of the gas and accumulate at the bottom of the tower even if the gas outlet is provided at the region of the bottom of the tower. This accumulation of settled dust tends to form because of the decrease in the velocity of the gas within the tower, resulting in a sedimentation effect, increase in the mass of particles by moisturization, partial agglomeration of particles and for numerous other reasons which are not material to the improvements to be described below.
The dust which settles at the base of the tower is customarily discharged therefrom by screw conveyors, lock chambers in which the lock compartments are defined by star wheels, and like discharge devices well known to the art.
However, the handling of thesettled dust which is thus removed from the tower poses significant problems. For example, if the moisture content of the dust is high, i.e., the dust is in the form of sludge, it cannot be readily stored or handled and hence must be subjected to drying and processing in separate equipment. It is impractical to reintroduce the moist dust into the tower since this tends to increase the agglomeration effect and presents difficulties with respect to distributing the dust uniformly in the tower and interferes with the moisturization of the gas. It has thus been the practice to simply discard the moist dust and accept the inconveniences which result therefrom.
Finally, in connection with the prior-art systems mention may be made of the fact that recycling of the moist dust or the sludge, and any procedures involved in the handling thereof remote from the cooling tower, have resulted in caking of the solids on the walls of the apparatus concerned, including the walls of the cooling tower, to the detriment of the process efficiency.